Last summer I installed a battery equalizer system on my 24V '89 BJ73. This allows 12 volt accessories to pull from the first battery in series while maintaining an equal charge on both batteries. I won't go into the science of why equal charge is important on 24V series systems, but it's worth a google search if you aren't familiar with the issue or consequences.
In my case, previous owner had skirted the issue by installing a cheap step down / buck converter. I discovered this device buried behind the dash after troubleshooting a severe parasitic drain. The converter was the cause - it was drawing current to "buck" power via induction, even when there was no load on the device. This is normal behavior for these things, and while it can be circumvented by installing a relay upstream of the converter, I wanted something that provided a 12 volt solution while actively equalizing the batteries. In the case of 12V radios, you need constant 12V power for standby current, so even with a switched converter, you'd still be pulling straight from battery #1 and creating an unbalance over enough time. After some research, I landed on the Vanner Voltmaster Battery Equalizer. I found a 60 amp model on eBay.
Battery Equalizer
I installed everything using steel DIN rail - it provided the right strength/rigidity, and also was adaptable enough to use with existing bolt holes on chassis. No holes were drilled! I had to get creative, bend one of the rails, and use some metal spacers. It was a tight fit but I think it turned out nicely. Wiring to batteries was done with 8 AWG cable and appropriate fuses for protection.
Solenoid Switches
Two solenoid switches are used - one for the 24V battery connection and one for 12V connection. Using only one on the 24, 12, or ground side won't help you because a complete, charged circuit remains after any one of those scenarios (that was a fun one to figure out!). I had to go with some lower profile, nylon solenoids because the traditional "trash can" style would not fit due to the size and stud locations. I hesitated to use relays due to the potential load on these things. They are 24V solenoids and are energized by ACC power run from the handy factory engine bay junction box (thank you Toyota). Again, DIN rail to the rescue.
And lastly, a pic of the entire system:
In my case, previous owner had skirted the issue by installing a cheap step down / buck converter. I discovered this device buried behind the dash after troubleshooting a severe parasitic drain. The converter was the cause - it was drawing current to "buck" power via induction, even when there was no load on the device. This is normal behavior for these things, and while it can be circumvented by installing a relay upstream of the converter, I wanted something that provided a 12 volt solution while actively equalizing the batteries. In the case of 12V radios, you need constant 12V power for standby current, so even with a switched converter, you'd still be pulling straight from battery #1 and creating an unbalance over enough time. After some research, I landed on the Vanner Voltmaster Battery Equalizer. I found a 60 amp model on eBay.
Battery Equalizer
I installed everything using steel DIN rail - it provided the right strength/rigidity, and also was adaptable enough to use with existing bolt holes on chassis. No holes were drilled! I had to get creative, bend one of the rails, and use some metal spacers. It was a tight fit but I think it turned out nicely. Wiring to batteries was done with 8 AWG cable and appropriate fuses for protection.
Solenoid Switches
Two solenoid switches are used - one for the 24V battery connection and one for 12V connection. Using only one on the 24, 12, or ground side won't help you because a complete, charged circuit remains after any one of those scenarios (that was a fun one to figure out!). I had to go with some lower profile, nylon solenoids because the traditional "trash can" style would not fit due to the size and stud locations. I hesitated to use relays due to the potential load on these things. They are 24V solenoids and are energized by ACC power run from the handy factory engine bay junction box (thank you Toyota). Again, DIN rail to the rescue.
And lastly, a pic of the entire system:
